You could try dosing some Epsom salts, that is magnesium something. Could research calcium, I think there is something you can buy at HD that has calcium, plaster of paris maybe? I happen to have calcium carbonate I used in homemade dog food, sure doesn't dissolve well though.

My Rotala was stunted and a bit of GH booster perked it up. Your plant isn't stunted though. What is the nitrate level?